HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission.
In order to use HVDC electrical power transmission, it is typically necessary to convert alternating current (AC) to direct current (DC) and back again. To date most HVDC transmission systems have been based on line commutated converters (LCCs), for example such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased. In LCCs the converter relies on the connected AC voltage to provide commutation from one valve to another.
Increasingly however voltage source converters (VSCs) are being proposed for use in HVDC transmission. HVDCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometime referred to as self-commutating converters.
VSCs typically comprise multiple converter arms, each of which connects one DC terminal to one AC terminal. For a typical three phase AC input/output there are six converter arms, with the two arms connecting a given AC terminal to the high and low DC terminals respectively forming a phase limb. Each converter arm comprises an apparatus which is commonly termed a valve and which typically comprises a plurality of elements which may be switched in a desired sequence.
In one form of known VSC, often referred to as a six pulse bridge, each valve comprises a set of series connected switching elements, typically insulated gate bipolar transistors (IGBTs) connected with respective antiparallel diodes. The IGBTs of the valve are switched together to electrically connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb typically being switched in anti-phase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved.
In high voltage applications where a large number of series connected IGBTs are required the approach does however require complex drive circuitry to ensure that the IGBTs switch at the same time as one another and requires large passive snubber components to ensure that the high voltage across the series connected IGBTs is shared correctly. In addition the IGBTs need to switch on and off several times over each cycle of the AC voltage frequency to control the harmonic currents. These factors can lead to relatively high losses in conversion, high levels of electromagnetic interference and a complex design.
In another known type of VSC, referred to a modular multilevel converter (MMC), each valve comprises a plurality of cells connected in series, each cell comprising an energy storage element such as a capacitor and a switch arrangement that can be controlled so as to either connect the energy storage element between the terminals of the cell or bypass the energy storage element. The cells are sometimes referred to as sub-modules, with a plurality of cells forming a module. The sub-modules of a valve are controlled to connect or bypass their respective energy storage elements at different times so as to vary over the time the voltage difference across the plurality of cells. By using a relatively large number of sub-modules and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a desired waveform, such as a sine wave, to convert from DC to AC or vice versa with low levels of harmonic distortion. As the various sub-modules are switched individually and the changes in voltage from switching an individual sub-module are relatively small a number of the problems associated with the six pulse bridge converter are avoided.
In the MMC design each valve is operated continually through the AC cycle with the two valves of a phase limb being switched in synchronism to provide the desired voltage waveform.
Recently a variant converter has been proposed wherein a series of connected cells is provided in a converter arm for providing a stepped voltage waveform as described, but each converter arm is turned off for at least part of the AC cycle. Thus the plurality of series connected cells for voltage wave-shaping are connected in series with switching elements which can be turned off when the relevant converter arm is in the off state and not conducting. Such a converter has been referred to as an Alternate-Arm-Converter (AAC). An example of such a converter is described in WO2010/149200.
FIG. 1 illustrates a known Alternate-Arm-Converter (AAC) 100. The example converter 100 has three phase limbs 101a-c, each phase limb having a high side converter arm connecting the relevant AC terminal 102a-c to the high side DC terminal DC+ and a low side converter arm connecting the relevant AC terminal 102a-c to the low side DC terminal DC−. Each converter arm comprises a circuit arrangement 103 of series connected cells, the arrangement 103 being in series with an arm switch 104 and inductances 105. It will be noted that FIG. 1 illustrates a single arm inductance but one skilled in the art will appreciated that the arm inductance may in practice be distributed along the arm between the AC and DC terminals.
The circuit arrangement 103 comprises a plurality of cells 106 connected in series. Each cell 106 has an energy storage element that can be selectively connected in series between the terminals of the cell or bypassed. In the example shown in FIG. 1 each cell 106 has terminals 107a, 107b for high-side and low-side connections respectively and comprises a capacitor 108 as an energy storage element. The capacitor 108 is connected with cell switching elements 109, e.g. IGBTs with antiparallel diodes, to allow the terminals 107a and 107b of the cell to be connected via a path that bypasses capacitor 108 or via a path that includes capacitor 108 connected in series. In the example illustrated in FIG. 1 each cell comprises four cell switching elements 109 in a full H-bridge arrangement such that the capacitor can be connected in use to provide either a positive or a negative voltage difference between the terminals 107a and 107b. In some embodiments however at least some of the cells may comprise switching elements in a half bridge arrangement such that the capacitor can be bypassed or connected to provide a voltage difference of a given polarity. The circuit arrangement 103 of such series connected cells can thus operate to provide a voltage level that can be varied over time to provide stepped voltage waveform for wave-shaping as discussed above. The circuit arrangement 103 is sometimes referred to as a chain-link circuit or chain-link converter or simply as a chain-link. In this disclosure the circuit arrangement 103 of such series connected cell for providing a controlled voltage shall be referred to as a chain-link.
In the AAC converter the chain-link 103 in each converter arm is connected in series with an arm switch 104, which will be referred to herein as a director switch, which may comprise a plurality of series connected arm switching elements 110. The director switch of an arm may for example comprise high voltage elements with turn-off capability such as IGBTs or the like with antiparallel diodes. When a particular converter arm is conducting, the chain-link 103 is switched in sequence to provide a desired waveform in a similar fashion as described above with respect to the MMC type converter. However in the AAC converter each of the converter arms of a phase limb is switched off for part of the AC cycle and during such a period the director switch 104 is turned off.
For example, the director switch of the high side arm of a phase limb may be switched on to be conducting during the positive half of the relevant AC cycle and switched off to be non-conducting during the negative half of the cycle, with the low side director switch being switched in antiphase. During the positive half of the cycle the chain-link 103 of the high side arm is switched in a sequence to provide desired wave-shaping. During the negative half of the cycle the chain-link 103 of the low side arm provides wave-shaping. In such a mode of operation there may be no or only a limited amount of overlap between periods of conduction of the director switches 104 of the high side and the low side.
In some modes of operation, referred to as an overlap mode of operation, the director switch of the high side may be conducting, with the chain-link 103 of the high side arm providing wave-shaping, for part of the negative cycle, e.g. for a certain phase angle before and after the positive half of the cycle. Likewise the director switch of the low side may be conducting, with the chain-link 103 of the low side arm providing wave-shaping, for part of the positive cycle. There is thus an extended period of overlap when the director switches of both arms are on. Such a mode of operation does require the chain-links 103 of each converter arm to be able to generate a voltage greater than half the total DC voltage of the VSC. In the overlap period the full DC voltage is supported by both the low-side and top-side chain-links, in other words the sum of the voltages of both chain-links equals the total DC voltage, although it will be understood that the chain-links may instantaneously be providing different voltages to one another.
It will be understood that in normal operation the director switches 104 are turned off at a point when there is no current flowing through the switch and also with no voltage difference across the switch. For example as mentioned the high side director switch may be turned off at the end of the positive half of the AC cycle when the current has dropped to zero and the chain-link 103 provides a voltage equal to the voltage magnitude of the high side DC terminal so that there is no voltage difference across the arm switch. In the overlap mode of operation the high side director switch is turned off during the negative part of the cycle when the current is flowing via the low side director switch. Similar considerations apply to the low side arm.
In some instances however it may be necessary to turn a director switch of a converter off at a point at which it is conducting significant current. This is known as hard switching. This could be for instance due to some abnormal operating conditions such as those caused by grid fault.
In a hard switching event therefore the switching elements 110 of the director switch 104 are commanded to open whilst there is current flowing through the director switch. This results in a rapid drop of current through the relevant converter arm, with an equally rapid rise in current through the other converter arm of the phase limb.
These rapid ramps in current will induce a voltage in the arm inductances 105, the magnitude of which is related to the arm inductance values and the magnitude of the current, as well as the rate of change. This induced voltage can lead to a voltage stress on the arm switching elements and potentially an overvoltage on the arm switching elements 110, e.g. the IGBTs. In some instances, the collector to emitter voltage VCE could reach fault-level values outside the Safe Operating Area (SOA) that could potentially destroy them. Moreover, the dynamic voltage sharing amongst all switching elements 110 forming the director switch 104 may not be equal, due to mismatches in the device characteristics and different propagation and delay times. Consequently, not only the voltage across the complete director switch could exceed the safe limits of operation, but also, some of the devices could take a higher share of that voltage.
Embodiments of the invention are directed at methods and apparatus for the control of a VSC that at least mitigate at least some of the issues noted above.
Thus according to the present invention there is provided a voltage source converter comprising: at least one phase limb comprising a high side converter arm connecting an AC terminal to a high side DC terminal and a low side converter arm connecting the AC terminal to a low side DC terminal; wherein each of the converter arms comprises a chain-link circuit in series with a director switch; wherein each chain-link circuit comprises a plurality of series connected cells that can be controllably switched to generate a controlled voltage across the chain-link circuit; and a controller configured to turn-off the director switch of a converter arm that is conducting current in response to a hard-switching request for a first phase limb; wherein, in response to a hard-switching request, the controller is configured to control the chain-link circuits of the first phase limb, at any point in a phase cycle, to control at least one of: a DC voltage across the director switch; and the current flowing through the director switch to a predetermined level before turning the director switch off.
Embodiments of the invention thus relate to a voltage source converter (VSC) of the Alternate-Arm-Converter (AAC) type having a controller for controlling hard switching of the VSC. In response to a hard switching request, i.e. a command to turn off the director switch of a converter arm that is conducting current, the controller controls the chain-link circuits of the phase limb so as to reduce at least one of the DC voltage across the director switch or the current flowing through the director switch before it is turned off. As will be explained in more detail below the DC voltage across the director switch is the component of the DC voltage between the DC terminals that would be experienced across the director switch if it were off. As will be explained in more detail later by reducing the DC voltage across the director switch and/or the current flowing through the switch before it is turned off, the voltage stresses experienced by the director switch as it turns off can be significantly reduced. It has been recognised that the chain-link circuits of the phase limb are controllable circuits that can be controlled relatively quickly in a hard switching event to reduce the voltage stresses on the director switch.
In some embodiments the predetermined level may substantially zero DC voltage and/or substantially zero current. In some embodiments however the predetermined level may be a relatively small non-zero voltage and/or current. The reference to controlling the DC voltage or current to the predetermined level shall be understood to mean controlling the voltage or current to be substantially no greater than the predetermined level. It shall also be understood that there may in some embodiments be different predetermined levels for voltage and for current.
In some embodiments the controller may be configured to control the chain-link circuits such that the combined voltage of the chain-link circuits of the first phase limb is equal to the DC voltage between the high side DC terminal and the low side DC terminal prior to turning the director switch off. In this way the chain-link circuits of the high side and low side arms offset the whole DC voltage and as a result there is no component to any voltage stress on the director switch as it opens that arises from the phase limb DC voltage.
The controller may be configured to determine whether the first phase limb is in an overlap state with the director switches of both converter arms on. In an overlap state the combined voltages of the high side and low side chain-link circuits will typically already be equal to the DC voltage and no further control may be needed. However if, the first phase limb is not in an overlap state, the controller may vary the voltage of at least one of the chain-link circuits, which may for example be the chain-link circuit of the converter arm that is conducting current.
In some embodiments the controller may configured to determine whether the first phase limb is within a predetermined time or phase angle of an overlap state, i.e. a certain time or phase angle before or after the start or end of an overlap period. If so the controller may be configured to control the chain-link circuits to start the overlap state early or extend the overlap state as appropriate. This can offset or eliminate any DC component on the director switch as it opens in way that reduces or minimises any disturbance of the AC voltage.
In some embodiments the controller may be further configured to control the voltage source converter to reduce the DC voltage between the high side DC terminal and the low side DC terminal period to turning the director switch off. This can make it easier for the chain-links to minimise the voltage stresses on the director switch subject to hard switching.
The controller may, in some embodiments, be further configured to control the chain-link circuits such that the combined voltage of the chain-link circuits of the first phase limb is stepped up for a period during turn-off of the director switch to at least partly counteract any voltage across the director switch due to induced voltages in inductances of the converter arms of the first phase limb. The combined voltage may be stepped up by an amount corresponding to (LARM++LARM−)·IP/tfall wherein LARM+ and LARM− are the inductance values of the high side and low side converter arm inductances respectively, IP is the current through the converter arm of the first phase limb prior to turning the director switch off and tfall is a predetermined time period related to the turn-off time of the director switch. The stepped-up voltage may be applied at a time to correspond to a ramp in current in the director switch as it turns off. In this embodiment it may be possible to ensure that there is substantially zero voltage stress on the director switch as it opens.
In some embodiments the controller may be further configured to control the voltage source converter to vary a modulation scheme for harmonic control prior to turning the director switch off. As will be described in more detail below in some embodiment modulations schemes, especially for controlling the harmonics, e.g. tripplen harmonic injection, may result in an increased voltage stress on a director switch during hard switching. The controller may therefore disable or modify such schemes during hard switching to mitigate such detrimental effects or even to provide a benefit.
In some embodiments the controller may configured to control the chain-link circuits to block the chain-link circuit in the converter arm that is conducting current before turning the director switch off. Blocking the chain-link circuit may comprise turning all of the switching elements of at least some of the cells of the chain-link circuit so as to introduce an energy storage element of the cell into the phase arm for charging.
Blocking the chain-link circuits can effectively cancel or reduce the DC voltage component across the director switch. Thus in some embodiments the chain-link circuit of the converter arm can be switched to a blocked state and then the director switch opening with the DC voltage across the switch as it opens being effectively zero.
In some embodiments the controller may be configured to control the chain-link circuits to reduce the arm current to the predetermined level before turning the director switch off.
In some embodiments the controller may configured to control the chain-link circuits to block the chain-link circuit in the converter arm that is conducting current in order to reduce the arm current to the predetermined level. As mentioned above blocking the chain-link circuit may comprise turning all of the switching elements of at least some of the cells of the chain-link circuit so as to introduce an energy storage element of the cell into the phase arm for charging. This causes the arm current to reduce when the director switch is still conducting and thus no significant voltage stress develops across the switch.
In other embodiments however the controller may be configured to control the chain-link circuits such that at least the chain-link circuit in the arm conducting current provides a voltage level that reduces the arm current. Thus, rather than block the chain-link circuit, it may be controlled to provide a voltage that results in a reduction of the arm current.
However the chain-link circuits are controlled to reduce the arm current, after the arm current has reached the predetermined level the director switch may be opened, i.e. turned off. In some embodiments however the controller may be configured such that, after the current has dropped to the predetermined level, control the chain-link circuits such that the combined voltage of the chain-link circuits of the first phase limb is equal to the DC voltage between the high side DC terminal and the low side DC terminal prior to turning the director switch off.
The controller of the VSC may be implemented by one or more modules that may comprise hardware or software or a combination of both.
The voltage source converter may form part of an HVDC power transmission or distribution network and another aspect of the invention relates to an HVDC power transmission or distribution network comprising a VSC as described above.
The invention also relates to a method of controlling a VSC. Thus in another aspect there is provided a method of controlling a voltage source converter having at least one phase limb comprising a high side converter arm connecting an AC terminal to a high side DC terminal and a low side converter arm connecting the AC terminal to a low side DC terminal; each of the converter arms comprising a chain-link circuit in series with a director switch, wherein each chain-link circuit comprises a plurality of series connected cells that can be controllably switched to generate a controlled voltage across the chain-link circuit. The method comprises: in response to a hard-switching request for a first phase limb at any point in a phase cycle, controlling the chain-link circuits of the first phase limb to control at least one of: a DC voltage across the director switch; and the current flowing through the director switch to a predetermined level; and subsequently turning the director switch off.
The method according to this aspect of the invention provides all of the same benefits and may be implemented in any of the variants as discussed above in relation to the first aspect of the invention.
In particular in some examples controlling the chain-link circuits of the first phase limb may comprise controlling the voltages of chain-link circuits such that the combined voltage of the chain-link circuits of the first phase limb is equal to the DC voltage between the high side DC terminal and the low side DC terminal prior to turning the director switch off. Additionally or alternatively in some examples controlling the chain-link circuits of the first phase limb may comprise blocking the chain-link circuit in the converter arm of the first phase limb that is conducting before turning the director switch off. In some examples controlling the chain-link circuits of the first phase limb may comprise controlling at least the chain-link circuit in the converter arm of the first phase limb that is conducting current to reduce the arm current to the predetermined level before turning the director switch off.
Aspects of the invention also relate to machine readable instructions, such as software code, comprising instructions for causing a suitable apparatus, such as a processor or controller of a VSC, to operate the methods described. The machine readable instructions may be stored on a non-transitory storage medium such as a memory of some sort.
In a further aspect the invention also relates to a controller of a VSC having: at least one phase limb comprising a high side converter arm connecting an AC terminal to a high side DC terminal and a low side converter arm connecting the AC terminal to a low side DC terminal; wherein each of the converter arms comprises a chain-link circuit in series with a director switch; wherein each chain-link circuit comprises a plurality of series connected cells that can be controllably switched to generate a controlled voltage across the chain-link circuit.
The controller is configured to turn-off the director switch of a converter arm that is conducting current in response to a hard-switching request for a first phase limb; wherein, in response to a hard-switching request, the controller is configured to control the chain-link circuits of the first phase limb, at any point in a phase cycle, to control at least one of: a DC voltage across the director switch; and the current flowing through the director switch to a predetermined level before turning the director switch off.